Electrostatic Changes in Lycopersicon esculentum Root Plasma Membrane Resulting from Salt Stress

Evaluating the resistance mechanism of Atriplex leucoclada (Orache) to salt and water stress; A potential crop for biosaline agriculture

The development of food and forage crops that flourish under saline conditions may be a prospective avenue for mitigating the impacts of climate change, both allowing biomass production under conditions of water-deficit and potentially expanding land-use to hitherto non-arable zones. Here, we examine responses of the native halophytic shrub Atriplex leucoclada to salt and drought stress using a factorial design, with four levels of salinity and four drought intensities under the arid conditions. A. leucoclada plants exhibited morphological and physiological adaptation to salt and water stress which had little effect on survival or growth. Under low salinity stress, water stress decreased the root length of A. leucoclada; in contrast, under highly saline conditions root length increased. Plant tissue total nitrogen, phosphorus and potassium content decreased with increasing water stress under low salinity. As salt stress increased, detrimental effects of water deficit diminished. We found that both salt and water stress had increased Na⁺ and Cl^- uptake, with both stresses having an additive and beneficial role in increasing ABA and proline content. We conclude that A. leucoclada accumulates high salt concentrations in its cellular vacuoles as a salinity resistance mechanism; this salt accumulation then becomes conducive to mitigation of water stress. Application of these mechanisms to other crops may improve tolerance and producitivity under salt and water stress, potentially improving food security.

Salt tolerance mechanisms in the Lycopersicon clade and their trade-offs

Salt stress impairs growth and yield in tomato, which is mostly cultivated in arid and semi-arid areas of the world. A number of wild tomato relatives (Solanum pimpinellifolium, S. pennellii, S. cheesmaniae and S. peruvianum) are endemic to arid coastal areas and able to withstand higher concentration of soil salt concentrations, making them a good genetic resource for breeding efforts aimed at improving salt tolerance and overall crop improvement. However, the complexity of salt stress response makes it difficult to introgress tolerance traits from wild relatives that could effectively increase tomato productivity under high soil salt concentrations. Under commercial production, biomass accumulation is key for high fruit yields, and salt tolerance management strategies should aim to maintain a favourable plant water and nutrient status. In this review, we first compare the effects of salt stress on the physiology of the domesticated tomato and its wild relatives. We then discuss physiological and energetic trade-offs for the different salt tolerance mechanisms found within the Lycopersicon clade, with a focus on the importance of root traits to sustain crop productivity.

Electrical impedance spectroscopy (EIS) in plant roots research: a review

Nondestructive testing of plant roots is a hot topic in recent years. The traditional measurement process is time-consuming and laborious, and it is impossible to analyze the state of plant roots without destroying the sample. Recent studies have shown that as an excellent nondestructive measurement method, although electrical impedance spectroscopy (EIS) has made great achievements in many botanical research fields such as plant morphology and stress resistance, there are still limitations. This review summarizes the application of EIS in plant root measurement. The experiment scheme, instrument and electrode, excitation frequency range, root electrical characteristics, equivalent circuit, and combination of EIS and artificial intelligence (AI) are discussed. Furthermore, the review suggests that future research should focus on miniaturization of measurement equipment, standardization of planting environment and intelligentization of root diagnosis, so as to better apply EIS technology to in situ root nondestructive measurement.

Sulfur nanoparticles mediated improvement of salt tolerance in wheat relates to decreasing oxidative stress and regulating metabolic activity

Salinity is a critical issue impairing the growth and productivity of most crop species through the mediated ionic and osmotic imbalances. As a way forward, the current study was tailored to elucidate the capacity of sulfur nanoparticles (SNPs) to amend salinity consequences on growth and physio-biochemical attributes of wheat. In a controlled experiment, wheat seeds were primed for 12 h with either 100 μM SNPs or deionized water then sown in plastic pots containing 5 kg clay-sand mixture (2:1 w/w). A week later, pots received NaCl (100 or 200 mM) as a sole treatment or in combination with SNPs and after three weeks the data of morph-bio-physiological traits were recorded. Salinity decreased growth rate, pigmentation, protein, amino acids, cysteine, ascorbate, flavonoids and phenolics content in wheat leaves. Plants pre-treated with 100 μM SNPs showed improved growth rate, pigmentation, nitrogen metabolism as well as non-enzymatic antioxidant contents as compared with salinized treatments. Neither salt nor SNP treatments affected photosynthetic performance rate (Fv/fm), however both treatments induced glutathione content. SNP treatment retrieved the undue excessive activities of catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), superoxide dismutase (SOD) and polyphenol oxidase (PPO) besides the increased level of proline caused by salt stress. Likewise, 100 μM SNPs rebalanced the declined nitrogen, phosphorus and potassium contents and decreased sodium uptake caused by salinity. On the whole, priming with 100 μM SNPs improved photosynthetic pigments, nitrogen metabolism, antioxidant status and ionic relations contributing to the enhancement of growth attributes in wheat under salinity.

Regulation of hormonal responses of sweet pepper as affected by salinity and elevated CO2 concentration

This study examines the extent to which the predicted CO2 -protective effects on the inhibition of growth, impairment of photosynthesis and nutrient imbalance caused by saline stress are mediated by an effective adaptation of the endogenous plant hormonal balance. Therefore, sweet pepper plants (Capsicum annuum, cv. Ciclón) were grown at ambient or elevated [CO2] (400 or 800 µmol mol(-1)) with a nutrient solution containing 0 or 80 mM NaCl. The results show that, under saline conditions, elevated [CO2] increased plant dry weight, leaf area, leaf relative water content and net photosynthesis compared with ambient [CO2], whilst the maximum potential quantum efficiency of photosystem II was not modified. In salt-stressed plants, elevated [CO2 ] increased leaf NO3(-) concentration and reduced Cl(-) concentration. Salinity stress induced ABA accumulation in the leaves but it was reduced in the roots at high [CO2], being correlated with the stomatal response. Under non-stressed conditions, IAA was dramatically reduced in the roots when high [CO2] was applied, which resulted in greater root DW and root respiration. Additionally, the observed high CK concentration in the roots (especially tZR) could prevent downregulation of photosynthesis at high [CO2], as the N level in the leaves was increased compared with the ambient [CO2], under salt-stress conditions. These results demonstrate that the hormonal balance was altered by the [CO2], which resulted in significant changes at the growth, gas exchange and nutritional levels.

Pretreated cheese whey wastewater management by agricultural reuse: chemical characterization and response of tomato plants Lycopersicon esculentum Mill. under salinity conditions

The agricultural reuse of pretreated industrial wastewater resulting from cheese manufacture is shown as a suitable option for its disposal and management. This alternative presents attractive advantages from the economic and pollution control viewpoints. Pretreated cheese whey wastewater (CWW) has high contents of biodegradable organic matter, salinity and nutrients, which are essential development factors for plants with moderate to elevated salinity tolerance. Five different pretreated CWW treatments (1.75 to 10.02 dS m(-1)) have been applied in the tomato plant growth. Fresh water was used as a control run (average salinity level=1.44 dS m(-1)). Chemical characterization and indicator ratios of the leaves, stems and roots were monitored. The sodium and potassium leaf concentrations increased linearly with the salinity level in both cultivars, Roma and Rio Grande. Similar results were found in the stem sodium content. However, the toxic sodium accumulations in the cv. Roma exceeded the values obtained in the cv. Rio Grande. In this last situation, K and Ca uptake, absorption, transport and accumulation capacities were presented as tolerance mechanisms for the osmotic potential regulation of the tissues and for the ion neutralization. Consequently, Na/Ca and Na/K ratios presented lower values in the cv. Rio Grande. Na/Ca ratio increased linearly with the salinity level in leaves and stems, regardless of the cultivar. Regarding the Na/K ratio, the values demonstrated competition phenomena between the ions for the cv. Rio Grande. Despite the high chloride content of the CWW, no significant differences were observed for this nutrient in the leaves and stems. Thus, no nitrogen deficiency was demonstrated by the interaction NO3(-)/Cl(-). Nitrogen also contributes to maintain the water potential difference between the tissues and the soil. Na, P, Cl and N radicular concentrations were maximized for high salinity levels (≥2.22 dS m(-1)) of the pretreated CWW.

Plant growth-promoting bacteria as a tool to improve salinity tolerance in sweet pepper

To characterise the effect of bacterial inoculants (Azospirillum brasilense and Pantoea dispersa) on the response of sweet pepper (Capsicum annuum L.) to saline stress, plants were exposed to 0, 40, 80 and 120mM NaCl in solution. The effect on plant growth; leaf gas exchange; NO3-, Cl-, K+ and Na+ accumulation; and chlorophyll fluorescence and content were investigated. Total plant DW was reduced significantly by salinity but when inoculants were applied, DW was increased. Inoculated plants showed higher DW accumulation in the roots. Salinity levels up to 80mM NaCl did not affect the net assimilation rate in inoculated plants but 40mM NaCl was enough to reduce this parameter in non-inoculated plants. The leaf area ratio was not modified substantially by inoculation. The leaf Cl- concentration of inoculated plants was reduced at the highest salinity, compared with control plants, and NO3- concentration increased markedly. A higher K+:Na+ ratio was found in inoculated plants. Leaf photosynthesis and stomatal conductance were impaired significantly at moderate, but not low, salinity, the effect of inoculation being enough to maintain higher stomatal conductance under higher stress. The photochemical efficiency of PSII and the relative chlorophyll content were not affected by the inoculants. Thus, the effects of the inoculants on the response to salinity were due mainly to stomatal regulation of photosynthesis rather than effects on biochemical limitations on photosynthesis. These results indicate the benefits of these bacterial inoculants in ameliorating the deleterious effect of NaCl in a salt-sensitive crop like sweet pepper.

Separating multiple, short-term, deleterious effects of saline solutions on the growth of cowpea seedlings

• Reductions in plant growth as a result of salinity are of global importance in natural and agricultural landscapes. • Short-term (48-h) solution culture experiments studied 404 treatments with seedlings of cowpea (Vigna unguiculata cv Caloona) to examine the multiple deleterious effects of calcium (Ca), magnesium (Mg), sodium (Na) or potassium (K). • Growth was poorly related to the ion activities in the bulk solution, but was closely related to the calculated activities at the outer surface of the plasma membrane, {I(z)}₀°. The addition of Mg, Na or K may induce Ca deficiency in roots by driving {Ca²+}₀° to < 1.6 mM. Shoots were more sensitive than roots to osmolarity. Specific ion toxicities reduced root elongation in the order Ca²+ > Mg²+ > Na+ > K+. The addition of K and, to a lesser extent, Ca alleviated the toxic effects of Na. Thus, Ca is essential but may also be intoxicating or ameliorative. • The data demonstrate that the short-term growth of cowpea seedlings in saline solutions may be limited by Ca deficiency, osmotic effects and specific ion toxicities, and K and Ca alleviate Na toxicity. A multiple regression model related root growth to osmolarity and {I(z)}₀° (R²=0.924), allowing the quantification of their effects.

Interactive effects of salinity and iron deficiency in Medicago ciliaris

In calcareous salt-affected soils, iron availability to plants is subjected to the effects of both sodium and bicarbonate ions. Our aim was to study interactive effects of salinity and iron deficiency on iron acquisition and root acidification induced by iron deficiency in Medicago ciliaris L., a species commonly found in saline ecosystems. Four treatments were used: C, control treatment, complete medium (CM) containing 30 microM Fe; S, salt treatment, CM with 75 mM NaCl; D, deficient treatment, CM containing only 1 microM Fe; DS, interactive treatment, CM containing 1 microM Fe with 75 mM NaCl. Our study showed that plant growth and chlorophyll content were much more affected by the interactive treatment than by iron deficiency or by the salt treatment, indicating an additive effect of these constraints in DS plants. These results could be partially explained by Na accumulation in shoots as well as a limitation of nutrient uptake such as Fe and K under salt stress, under iron deficiency, and especially under their combined effect. The study also showed that root acidification was deeply diminished when iron deficiency was associated with salinity. This probably explained the decrease of Fe uptake and suggested that root proton pump activity would be inhibited by salinity.

The role of the plasma membrane in the response of plant roots to aluminum toxicity

Al(3+), the predominant form of solubilized aluminum at pH values below 5.0, has been shown to exert a profound inhibitory effect on root elongation. Al is known to accumulate at the root apex. The plasma membrane represents the first potential target for Al toxicity, due to its pronounced binding to phospholipids. Al appears to alter both the structure and functions of the plasma membrane, and a great deal of research has been conducted concerning the interactions between Al and the plasma membrane. In this review, recent findings regarding the interactions between Al and the plasma membrane are described, specifically findings involving Al-induced alterations in the structure and function of the plasma membrane.

Salinity and the growth of non-halophytic grass leaves: the role of mineral nutrient distribution

Salinity is increasingly limiting the production of graminaceous crops constituting the main sources of staple food (rice, wheat, barley, maize and sorghum), primarily through reductions in the expansion and photosynthetic yield of the leaves. In the present review, we summarise current knowledge of the characteristics of the spatial distribution patterns of the mineral elements along the growing grass leaf and of the impact of salinity on these patterns. Although mineral nutrients have a wide range of functions in plant tissues, their functions may differ between growing and non-growing parts of the grass leaf. To identify the physiological processes by which salinity affects leaf elongation in non-halophytic grasses, patterns of mineral nutrient deposition related to developmental and anatomical gradients along the growing grass leaf are discussed. The hypothesis that a causal link exists between ion deficiency and / or toxicity and the inhibition of leaf growth of grasses in a saline environment is tested.

Electrical potentials of plant cell walls in response to the ionic environment

Electrical potentials in cell walls (psi(Wall)) and at plasma membrane surfaces (psi(PM)) are determinants of ion activities in these phases. The psi(PM) plays a demonstrated role in ion uptake and intoxication, but a comprehensive electrostatic theory of plant-ion interactions will require further understanding of psi(Wall). psi(Wall) from potato (Solanum tuberosum) tubers and wheat (Triticum aestivum) roots was monitored in response to ionic changes by placing glass microelectrodes against cell surfaces. Cations reduced the negativity of psi(Wall) with effectiveness in the order Al(3+) > La(3+) > H(+) > Cu(2+) > Ni(2+) > Ca(2+) > Co(2+) > Cd(2+) > Mg(2+) > Zn(2+) > hexamethonium(2+) > Rb(+) > K(+) > Cs(+) > Na(+). This order resembles substantially the order of plant-root intoxicating effectiveness and indicates a role for both ion charge and size. Our measurements were combined with the few published measurements of psi(Wall), and all were considered in terms of a model composed of Donnan theory and ion binding. Measured and model-computed values for psi(Wall) were in close agreement, usually, and we consider psi(Wall) to be at least proportional to the actual Donnan potentials. psi(Wall) and psi(PM) display similar trends in their responses to ionic solutes, but ions appear to bind more strongly to plasma membrane sites than to readily accessible cell wall sites. psi(Wall) is involved in swelling and extension capabilities of the cell wall lattice and thus may play a role in pectin bonding, texture, and intercellular adhesion.

Role of dynamics of intracellular calcium in aluminium-toxicity syndrome

This review is concentrating on the role of aluminium (Al)-calcium (Ca) interactions in Al toxicity syndrome in plants. Disruption of cytoplasmic Ca²⁺ homeostasis has been suggested as a primary trigger of Al toxicity. Aluminium causes an increase in cytosolic Ca²⁺ activity, potentially disrupting numerous biochemical and physiological processes, including those involved in the root growth. The source of Ca²⁺ for the increase in cytosolic Ca²⁺ activity under Al exposure is partly extracellular (likely to be due to the Al-resistant portion of the flux through depolarization-activated Ca²⁺ channels and fluxes through Ca²⁺ -permeable nonselective cation channels in the plasma membrane) as well as intracellular (increased cytosolic Ca²⁺ activity enhances the activity of Ca²⁺ release channels in the tonoplast and the endoplasmic reticulum membrane). The effect on increased cytosolic Ca²⁺ activity of possible Al-related inhibition of the plasma membrane and endo-membrane Ca²⁺ -ATPases and Ca²⁺ exchangers (CaX) that sequester Ca²⁺ out of the cytosol is insufficiently documented at present. The relationship between Al toxicity, cytoplasmic Ca²⁺ homeostasis and cytoplasmic pH needs to be elucidated. Technical improvements that would allow measurements of cytosolic Ca²⁺ activity within the short time after exposure to Al (seconds or shorter) are eagerly awaited. Contents I. Introduction 296 II. Symptoms of aluminium toxicity 296 III. Calcium - aluminium interactions 297 IV. The role of electrical properties of the plasma membrane in calcium-aluminium interactions 306 V. Oxidative stress 307 VI. Callose 308 VII. Cytoskeleton 308 VIII. Conclusions 309 Acknowledgements 309 References 309.

In vivo and in vitro effects of cadmium on H+ ATPase activity of plasma membrane vesicles from oat (Avena sativa L.) roots

The effect of an in vivo and in vitro treatment with cadmium on transport activities of root plasma membrane enriched vesicles was studied in oat (Avena sativa L. cv. Argentina) plants. Addition of 100 mumol/L CdSO4 to nutrient solution decreases both proton transport activity and ATPase activity to the same level. In vitro experiments show that cadmium seems to have a differential inhibiting effect on proton transport activity and ATPase activity, the most pronounced one on ATP-dependent H(+)-accumulation, suggesting that cadmium would interfere with membrane permeability properties. This is indeed the case. The results demonstrate that cadmium decreases passive permeability to protons.

Transmembrane topography of plasma membrane constituents in mung bean (Vigna radiata L.) hypocotyl cells. II. The large scale asymmetry of surface peptides

The large scale asymmetry in surface (poly)peptides of the plasma membrane (PM) of mung bean (Vigna radiata L.) hypocotyl cells was investigated by protease and 1 M KCl treatments of PM vesicles obtained by an aqueous two-phase partition technique. Proteases only slightly reduced the protein content of right-side-out PM vesicles and the treatment with 1 M KCl resulted in the dissociation of only a few peripheral proteins from the outer surface of right-side-out PM vesicles, indicating that few surface peptides including peripheral proteins existed on the outer surface. From experiments of the re-partitioning of endomembrane vesicles removed from surface peptides, it was found that the surface peptide content is a factor determining the partitioning, and the hypothesis that sterols are asymmetrically distributed across higher plant PM was proposed. We speculate that asymmetrical properties between the outer and the inner surfaces of plant PM, especially in partitioning in the two-phase system, derive from the asymmetry of the bulk of surface peptides and PM sterols. The comparatively low hydrophilicity of the outer surface of the PM would be important for the partitioning of right-side-out PM vesicles in the upper phase of the two-phase system.

Aluminum inhibits the H(+)-ATPase activity by permanently altering the plasma membrane surface potentials in squash roots

Although aluminum (AL) toxicity has been widely studied in monocotyledonous crop plants, the mechanism of Al impact on economically important dicotyledonous plants is poorly understood. Here, we report the spatial pattern of Al-induced root growth inhibition, which is closely associated with inhibition of H(+)-ATPase activity coupled with decreased surface negativity of plasma membrane (PM) vesicles isolated from apical 5-mm root segments of squash (Cucurbita pepo L. cv Tetsukabuto) plants. High-sensitivity growth measurements indicated that the central elongation zone, located 2 to 4 mm from the tip, was preferentially inhibited where high Al accumulation was found. The highest positive shifts (depolarization) in zeta potential of the isolated PM vesicles from 0- to 5-mm regions of Al-treated roots were corresponded to pronounced inhibition of H(+)-ATPase activity. The depolarization of PM vesicles isolated from Al-treated roots in response to added Al in vitro was less than that of control roots, suggesting, particularly in the first 5-mm root apex, a tight Al binding to PM target sites or irreversible alteration of PM properties upon Al treatment to intact plants. In line with these data, immunolocalization of H(+)-ATPase revealed decreases in tissue-specific H(+)-ATPase in the epidermal and cortex cells (2--3 mm from tip) following Al treatments. Our report provides the first circumstantial evidence for a zone-specific depolarization of PM surface potential coupled with inhibition of H(+)-ATPase activity. These effects may indicate a direct Al interaction with H(+)-ATPase from the cytoplasmic side of the PM.

Computation of surface electrical potentials of plant cell membranes . Correspondence To published zeta potentials from diverse plant sources

A Gouy-Chapman-Stern model has been developed for the computation of surface electrical potential (psi0) of plant cell membranes in response to ionic solutes. The present model is a modification of an earlier version developed to compute the sorption of ions by wheat (Triticum aestivum L. cv Scout 66) root plasma membranes. A single set of model parameters generates values for psi0 that correlate highly with published zeta potentials of protoplasts and plasma membrane vesicles from diverse plant sources. The model assumes ion binding to a negatively charged site (R- = 0.3074 &mgr;mol m-2) and to a neutral site (P0 = 2.4 &mgr;mol m-2) according to the reactions R- + IZ &rlharr; RIZ-1 and P0 + IZ &rlharr; PIZ, where IZ represents an ion of charge Z. Binding constants for the negative site are 21, 500 M-1 for H+, 20,000 M-1 for Al3+, 2,200 M-1 for La3+, 30 M-1 for Ca2+ and Mg2+, and 1 M-1 for Na+ and K+. Binding constants for the neutral site are 1/180 the value for binding to the negative site. Ion activities at the membrane surface, computed on the basis of psi0, appear to determine many aspects of plant-mineral interactions, including mineral nutrition and the induction and alleviation of mineral toxicities, according to previous and ongoing studies. A computer program with instructions for the computation of psi0, ion binding, ion concentrations, and ion activities at membrane surfaces may be requested from the authors.

Three mechanisms for the calcium alleviation of mineral toxicities

Ca2+ in rooting medium is essential for root elongation, even in the absence of added toxicants. In the presence of rhizotoxic levels of Al3+, H+, or Na+ (or other cationic toxicants), supplementation of the medium with higher levels of Ca2+ alleviates growth inhibition. Experiments to determine the mechanisms of alleviation entailed measurements of root elongation in wheat (Triticum aestivum L. cv Scout 66) seedlings in controlled medium. A Gouy-Chapman-Stern model was used to compute the electrical potentials and the activities of ions at the root-cell plasma membrane surfaces. Analysis of root elongation relative to the computed surface activities of ions revealed three separate mechanisms of Ca2+ alleviation. Mechanism I is the displacement of cell-surface toxicant by the Ca2+-induced reduction in cell-surface negativity. Mechanism II is the restoration of Ca2+ at the cell surface if the surface Ca2+ has been reduced by the toxicant to growth-limiting levels. Mechanism III is the collective ameliorative effect of Ca2+ beyond mechanisms I and II, and may involve Ca2+-toxicant interactions at the cell surface other than the displacement interactions of mechanisms I and II. Mechanism I operated in the alleviation of all of the tested toxicities; mechanism II was generally a minor component of alleviation; and mechanism III was toxicant specific and operated strongly in the alleviation of Na+ toxicity, moderately in the alleviation of H+ toxicity, and not at all in the alleviation of Al3+ toxicity.